Beyond “Fermi’s Paradox” II: Questioning the Hart-Tipler Conjecture | Universe Today

Beyond “Fermi’s Paradox” II: Questioning the Hart-Tipler Conjecture

It’s become a legend of the space age. The brilliant physicist Enrico Fermi, during a lunchtime conversation at Los Alamos National Laboratory in 1950, is supposed to have posed a conundrum for proponents of the existence of extraterrestrial civilizations.

If space traveling aliens exist, so the argument goes, they would spread through the galaxy, colonizing every habitable world. They should then have colonized Earth. They should be here, but because they aren’t, they must not exist.

This is the argument that has come to be known as “Fermi’s paradox”. The problem is, as we saw in the first installment, Fermi never made it. As his surviving lunch companions recall (Fermi himself died of cancer just four years later, and never published anything on the topic of extraterrestrial intelligence), he simply raised a question, “Where is everybody?” to which there are many possible answers.

Fermi didn’t doubt that extraterrestrial civilizations might exist, but supposed that interstellar travel wasn’t feasible or that alien travelers had simply never found Earth in the vastness of the galaxy.

The argument claiming that extraterrestrials don’t exist was actually proposed by the astronomer Michael Hart, in a paper he published in 1975. Hart supposed that if an extraterrestrial civilization arose in the galaxy it would develop interstellar travel and launch colonizing expeditions to nearby stars. These colonies would, in turn, launch their own starships spreading a wave of colonization across the galaxy.

How long would the wave take to cross the galaxy? Assuming that the starships traveled at one tenth the speed of light and that no time was lost in building new ships upon arriving at the destination, the wave, Hart surmised, could cross the galaxy in 650,000 years.

Even allowing for a modicum of time for each colony to establish itself before building more ships, the galaxy could be crossed in two million years, a miniscule interval on a cosmic or evolutionary timescale. Hart asserted that because extraterrestrials aren’t already here on Earth, none exist in our galaxy.

Hart’s argument was extended by cosmologist Frank Tipler in 1980. Tipler supposed that alien colonists would be assisted by self-reproducing robots. His conclusion was announced in the title of his paper ‘Extraterrestrial intelligent beings do not exist’.

Why is it important that Hart’s argument wasn’t really also formulated by the eminent Enrico Fermi? Because Fermi’s name lends a credibility to the argument that it might not deserve. Supporters of the search for extraterrestrial intelligence (SETI) want to search for evidence that alien civilizations exist by using radio telescopes to listen for radio messages that extraterrestrials may have transmitted into space. Interstellar signaling is vastly cheaper than a starship, and is feasible with technology we have today.

Hart drew public policy consequences from his argument that extraterrestrials don’t exist. His paper concluded that “an extensive search for radio messages from other civilizations is probably a waste of time and money”.

Our political leaders heeded Hart’s advice. When Senator William Proxmire led the successful drive to kill funding for NASA’s fledgling SETI program in 1981, he used the Hart-Tipler argument. A second NASA SETI effort was scuttled by congress in 1993, and no public money has been allocated to the search for extraterrestrial radio signals ever since.

The Arecibo Radio Telescope in Puerto Rico was the site of NASA’s High Resolution Microwave Survey, a search for extraterrestrial radio messages. Funding was cut off for the project in 1993 following criticism in congress. Credit: Unites States National Science Foundation

Just how convincing is the Hart-Tipler conjecture? Like Hart, Carl Sagan was an optimist about the prospects for interstellar travel, and Sagan published his analysis of the consequences of interstellar travel for extraterrestrial intelligence a whole decade earlier than Hart, in 1963. Sagan and his co-author, the Russian astronomer Iosef Shklovskii devoted a chapter to the topic in their 1966 classic Intelligent Life in the Universe.

Like Hart, Sagan concluded that “if colonization is the rule, then even one spacefaring civilization would rapidly spread, in a time much shorter than the age of the galaxy, throughout the Milky Way. There would be colonies of colonies of colonies…”. So why didn’t Sagan, like Hart, assert that extraterrestrials don’t exist because they aren’t already here?

The answer is that Sagan, unlike Hart, considered unlimited colonization as only one of many possible ways that extraterrestrial spacefarers might act. He wrote that “habitable planets lacking technical civilizations will frequently be encountered by spacefaring civilizations. It is not clear what their response will be…Perhaps strict injunctions against colonization of populated but pre-technical planets are in effect in some Codex Galactica. But we are in no position to judge extraterrestrial ethics. Perhaps attempts are made to colonize every habitable planet…A whole spectrum of intermediate cases can also be imagined”.

Besides assuming that interstellar travel is feasible, Hart’s argument is based on very specific and highly speculative ideas about how extraterrestrials must behave. He assumed that they would pursue a policy of unlimited expansion, that they would expand quickly, and that once their colonies were established, they would last for millions or even billions of years. If any of his speculations about how extraterrestrials will act aren’t right, then his argument that they don’t exist fails.

The evolutionary biologist Stephen Jay Gould was scathing in his criticism of Hart’s speculation. He wrote that ”I must confess that I simply don’t know how to react to such arguments. I have enough trouble predicting the plans and reactions of the people closest to me. I am usually baffled by the thoughts and accomplishments of humans in different cultures. I’ll be damned if I can state with certainty what some extraterrestrial source of intelligence might do”.

In 1981, Sagan and planetary scientist William Newman published a response to Hart and Tipler. While Hart used a very simple mathematical argument, assuming that an alien civilization would spread almost as fast as its ships could travel, Newman and Sagan used a mathematical model like the ones that population biologists use to analyze the spread of animal populations to model interstellar colonization.

They concluded that the rates of expansion assumed by Hart are highly unrealistic. Expansion will be drastically slower, for example, if civilizations control their population growth rates on any given planet to avoid ecological collapse, if colonies have a finite life span, and if alien societies eventually outgrow expansionist tendencies. Hart’s assumption that an alien civilization would spread almost as fast as its ships can travel isn’t plausible. It’s possible to walk across Rome in a day, Newman and Sagan noted, but Rome wasn’t built in a day. It grew much more slowly.

If the evolution of intelligent life is at all likely, other civilizations could emerge before any hypothetical first wave of expansion swept slowly over the galaxy. If several worlds produced waves of colonization, they might encounter one another. What would happen then? Nobody knows. The history of the galaxy can’t be predicted from a few equations.

For Newman and Sagan, the absence of extraterrestrials on Earth doesn’t mean that they don’t exist elsewhere in the galaxy, or that they never launch starships. It just means that they don’t behave in the way Hart expected. They conclude that “except possibly in the very early history of the Galaxy, there are no very old galactic civilizations with a consistent policy of conquest of inhabited worlds; there is no Galactic Empire”.

So, Enrico Fermi never did produce a powerful argument that extraterrestrial intelligence probably doesn’t exist. Neither did Michael Hart. The simple truth is that nobody knows whether or not extraterrestrials exist in the galaxy. If they do exist though, it’s possible that discovering their radio messages would give us the evidence we need. Then we could stop speculating and start learning something.

References and Further Reading:

F. Cain (2013) Where are all the aliens? The Fermi paradox, Universe Today.

F. Cain (2014) Are intelligent civilizations doomed? Universe Today.

R. H. Gray (2012) The Elusive WOW, Searching for Extraterrestrial Intelligence, Palmer Square Press, Chicago, Illinois.

R. H. Gray (2015) The Fermi Paradox is neither Fermi’s nor a paradox, Astrobiology, 15(3): 195-199.

M. H. Hart, (1975) An explanation for the absence of extraterrestrials on Earth, Quarterly Journal of the Royal Astronomical Society, 16:128-135.

W. I. Newman and C. Sagan (1981) Galactic civilizations: Population dynamics and interstellar diffusion, Icarus, 46:293-327.

C. Sagan (1963) Direct contact among galactic civilizations by relativistic interstellar spaceflight, Planetary and Space Science, 11:485-489.

I. S. Shklovskii and C. Sagan (1966) Intelligent Life in the Universe. Delta Publishing Company, Inc. New York, NY.

F. Tipler (1980) Extraterrestrial intelligent beings do not exist, Quarterly Journal of the Royal Astronomical Society, 21:267-281.

S. Webb (2010) If the Universe is Teeming with Aliens…Where is Everybody? Fifty Solutions to the Fermi Paradox and the Problem of Extraterrestrial Life. Copernicus Books, New York, NY.

Paul Patton

Paul Patton is a freelance science writer and a doctoral student at the Institute for the History and Philosophy of Science and Technology at the University of Toronto. He also holds a doctorate in Neuroscience and has conducted research on how brains integrate information from multiple senses, and on the sensory systems of blind cavefish. He has been interested in space, astronomy, and extraterrestrial life and intelligence since early childhood.

View Comments

  • Let’s take a more expanded viewpoint. Let’s assume evolution works on more than just a matter substrate. That’s not hard considering it only requires the substrate to be able to form patterns that exist long enough for random reproduction processes to occur on them. So there could be life forms in dark matter and possibly dark energy. Indeed there could be an evolutionary process on the energy from which the universe came into being. While information in this universe seems to be restricted to move at c or less, this may be a problem only where there is a higgs field or for information encoded on a substrate interacting with the higgs field. And the latter may only be a problem for beings of a substrate restricted to the (3, -1) Minkowski half space (assuming it is embedded in a larger dimensional space). Given the possibility that life forms evolved on energy forms other than matter is there any evidence for that? I propose that’s the basis of many “religions”.
    Allowing a correspondence between dark energy, dark matter and angles, jinn and an intelligence overlaying the energy field from which all existence arises then there is a context for a passage in the Quran. O company of jinn and mankind, if you are able to pass beyond the regions of the heavens and the earth, then pass. You will not pass except by authority [from Allah ]. 55:33 This passage seems to imply the encompassing intelligence doesn’t allow interstellar migration without some from of permission.
    It may be we should be thankful that beings like ourselves, expansionist, belligerent, colonizing aren’t allowed interstellar travel. On the other hand, this viewpoint would imply we are already “visited” by “aliens”, we just don’t see them because of the “dark” nature of their substrate.

  • All you can say for certain is that if there are intelligent civilizations in the galaxy that they aren't expansionist. And I would say we are fortunate in that respect. The more sophisticated and refined a culture gets, the higher its standards are likely to be. The higher these standards are, the less likely it might become for branches to break off from the home planets. Colonization is likely to be a grueling and Spartan affair, and if home is quite comfortable, who would want to leave?
    The answer is likely to be that they would leave out of necessity, such as when their star becomes unstable and that could take billions of years. It would make sense to take to the stars to gather resources, but that may not require colonizing habitable planets, just stripping non-habitable ones and transporting them back. Looking at migrations and conquests in Earth history, these usually took place due to conditions becoming intolerable in a tribe's homeland forcing them to take to the sword and conquer another. Such migrant civilizations are likely to be trouble and could be us if we take to the stars while still starving many people of resources in favor of others.

  • If an Alien race has spread across the Galaxy on different Planets they would all evolve to the conditions on each of those Planets and after time will not look like the original race and could look totally different from each other but with the same DNA...

  • All our speculations about "extraterrestrial life" are mere what in shrink jargon is called 'Projection'. We tend to fit unknowns into our known model something that should be avoided if we do not want shocking surprises.

  • This is certainly the most interesting article and discussion UT has produced in a long time..

    @lapsed. Your arguments are very, very compelling. I was completely convinced by the Sagan arguments until i read your counterarguments.

    If you are indeed true this could have implications beyond the search for life. It would suggest that NO ONE in the galaxy EVER produced AI that POPULATE or EXPLORE the galaxy. This is in my view the most puzzling aspect believers in Sagan would have to explain. We shouldn't judge humanities expansion by our footprints on the Moon but by the spread of the Voyagers.

    It would also suggest that our doom due to ever expanding AI is highly unlikely. Maybe true AI is just not possible? So no Matrix/Terminator?

  • At this point everything is a guess. We simply don't know whether or not other intelligent space faring species exist. To assume they don't exist because they aren't here, and act as if that is true, is foolish in my opinion. That conclusion stops us from looking, and it is in the looking that we learn things. We may very well be alone, but I suspect we are not.

    My humble, uneducated, unknowing opinion is a space faring species starts by colonizing their own system. In doing that they realize the most efficient way to expand is not by colonizing other planets, but by building movable space colonies, that can get to an almost infinite source of resources and energy, not on planets, but on comets, asteroids, ... smaller objects without huge gravity wells. Building space colonies also allows the civilization to taylor the colony environment to it's hereditary needs, rather than spend enormous amounts of energy getting in and out of giant gravity wells on planets that may or may not be suitable.

    As for looking for communication by radio waves, focused light beams or any other speed of light method, I think this is a dead end. Any advanced civilization would probably find a more efficient less costly way to communicate. I think we are close to coming up with instantaneous communication through some sort of quantum teleportation, and once that happens, there will be no detectable signal, and we'll be communicating instantly no matter what the distance.

    Seti might find evidence of life by looking at planetary atmospheres, or might accidently pick up a signal of some sort, but I doubt if it will ever find a signal beamed at us at light speed, for the purpose of communicating with us. That doesn't mean we should reach a conclusion about what an alien species would or would not do, if it exists, and stop looking.

  • Two very important things about intelligent life.

    1) We could be the first intelligent life to fire a rocket into space. So the question is why aren't we spread though out the galaxy yet? Time. It takes a long time even to colonize another planet or moon in our system. And we expect an alien life to have done this in a few years?

    2) We so far have avoided making ourselves extinct when we first figured out how to create fission and later fusion. Maybe the natural course of civilizations at this point is for themselves to kill themselves. We aren't past that hump yet and still could achieve self-extinction.

    • Presumably we came very close to "achieving self-extinction" with the Cuban Missile Crisis. If Kennedy had been less self-reflective ala The Bush's, this could have been the Planet of the Cockroaches. And I suspect we are heading in that direction. As the west eventually ethically cleanses the last continent they will likely turn on themselves again.

  • 1 of the things that always leaves me 'scratching my head' is communication. We 'assume' that Radio Signals is the only way it is possible.


    Look at our history of communication:
    ... Flags symbols and colors.
    ... Morse Code, via Signal Lights
    ... Smoke Signals
    ... Drums
    ... Music
    ... Words - ( Written )
    ... Cryptography .. in all kinds.
    ... Computer Compression Message.
    ... and the list goes on.

    So what if an alien race evolved out of "Radio Signals" as we understand them in 2015 and are 1,000 of light years ahead of us?

    Just like the article there is two sides of the "alien life exists" - there are more then 2 sides of possible ways to communicate in space. Space if very big, and what IF 'they' figured out a much simpler and time effective way to send Space Messages?

    Here we are, look for "radio signals" - when they left that when we were still 'pounding on rocks' .....

    • I've always found it puzzling why someone with the technology for interstellar travel would still communicate via crop circles. Whereas humans were already using more efficient techniques like drums and smoke signals when we were still in the stone age!

      • I am confused by the comparison; is communicating with smoke and noise more impressive than crushing wheat in patterns? Maybe the aliens think we are that's all we are capable of understanding and that we respond to pretty pictures!

  • IMO, the six policy steps for building a Universal Peace Building Initiative are: (1) end of war rhetoric and replacement with peace building rhetoric e.g. the solution to climate change is not a "fight" - it is an adaptation; (2) support of all of everyone's human rights vis a vis the UN's Universal Human Rights circa 1948 ; (3) tabula rasa diplomacy i.e. diplomacy without historical precedent, preconditions or preconceptions; (4) cooperative prosperity e.g. sustainable economies based on renewable energy; profit sharing; jobs and (5,6) - conversion of militaries to first responders to natural disasters and intel agencies to account for foreign aid.

  • Extraterrestrial civilizations that survive and tender their planet for millions of years imply use of renewable sources of energy, energy conservation, terraforming, climate engineering and a peace building initiative that has replaced war. I am fairly well disgusted with Humanity's choice and commitment to war and nonrenewable energy and currently, I see no reason why Extraterrestrials would want direct contact without first monitoring our planet. Finally, these discussions usually exclude the UFO observations - "If extraterrestrial civilizations exist and have the capability to reach us, their motivation might be to monitor our planet because of concerns raised about human behavior." - Jean-Jacque Velasco from France and the UFO Question, excerpted from - UFOs: Generals, Pilots and Government Officials Go on the Record by Leslie Kean

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